A digital versatile disk player (DVD player) is an example of a conventional data player. In a DVD player, data written on a disk (DVD) is encrypted so that a player which does not adopt means for copyright protection cannot play the data on the disk and, therefore, only a player which is legally provided with a key can play the disk. Generally, the key required for playback is given to each DVD player.
Hereinafter, an example of a conventional data player will be described with reference to FIG. 18.
FIG. 18 is a block diagram illustrating an example of a conventional data player.
In FIG. 18, reference numeral 1000 designates a key storage unit which stores a contents key for decoding encrypted contents 1202 recorded on a digital medium 1200, and reference numeral 1100 designates a data player. The key storage unit 1000 comprises a controller 1001 and a key storage 1002 which stores the contents key. The data player comprises a controller 1101, a contents key holder 1102 which takes the contents key from the key storage unit 1000 and holds it, a reader 1104 which reads the encrypted contents 1202 from the digital medium 1200, an encrypted contents decoder 1105 which decodes the encrypted contents 1202, a player 1106 which plays the decoded contents, a signal output unit 1107 which outputs the contents to the outside, and a user operation input unit 1108 which transmits an instruction from the user to the controller 1101.
Hereinafter, a description will be given of the operation of the data player 1100 in the case where the user instructs the data player 1100 to play the contents of the digital medium 1200, through the user operation input unit 1108.
Before playing the encrypted contents 1202, the data player 1100 communicates with the controller 1001 of the key storage unit 1000 using the controller 1101, and proves that it is eligible to play the encrypted contents 1202 in the digital medium 1200. When the controller 1001 of the key storage unit 1000 confirms that the data player 1100 is eligible for playback, the data player 1100 receives the contents key from the key storage unit 1000 and stores it in the contents key holder 1102. Thereafter, the reader 1104 reads the encrypted contents 1202 from the digital medium 1200, and the decoder 1105 decodes the encrypted contents 1202 using the contents key which has been stored, thereby obtaining plaintext contents. The plaintext contents are played by the player 1106 and then outputted as a signal by the signal output unit 1107. In this construction, when performing playback of specific encrypted contents existing on a digital medium, the data player 1100 is confirmed by the key storage unit 1000 as to whether it is eligible for playing the encrypted contents or not. Thereby, the encrypted contents on the digital medium are prevented from being played by an illegal player which is not eligible for playback.
On the other hand, with respect to electronic distribution of movies to theaters or video services in airplanes, there are some movies which have not yet been released in theaters and, therefore, it is desired that the periods during which these movies can be played are controllable. As a system for accessing such data, Japanese Published Patent Application No. Hei. 10-341212 discloses an encryption text transmission system. In this system, a key is provided with data about its available period or place, and the present time or position is detected when using the key, whereby the use of the key can be controlled according to whether the present time is within the available period or whether the present position where the key is used is within the available place. Further, the use of the key can be controlled by periodically recording time data and comparing the time data with the present time.
Further, it has been proposed that, when playing data, the ID number of a player which plays the data is embedded in the played data. Thereby, if the played data is illegally copied and sold, a player which assisted the illegal act can be identified.
In the conventional data player, however, once a data player is recognized as being eligible for playing specific contents by the key storage unit and then obtains a contents key and stores it in the contents key storage, the player can illegally play other encrypted contents by using the stored contents key even when the player is not eligible to play the contents.
Further, in the method disclosed in Japanese Published Patent Application No. Hei. 10-341212, since he time data or position data is judged after transmitting the key, to the player end, there is a risk that the key is obtained illegally. Further, there is a risk that the recorded time data is altered and the key is obtained illegally.
Furthermore, in the case where the data such as the player's ID number is embedded in the played data, the embedded data must be detected correctly. However, when the number of embedded data increases, it is difficult to detect all of them correctly and, further, there is no means to confirm whether the result of detection is correct or not.